Removing calcium sulfate scale



Patented Aug. 5, 1952 UNITED STATES -fjz toalsv REMOVINQ CALCIUM feM I JATE. s eepe Paul H. Cardwell and Billy P. -Robinson',Tulsa," I;

()kla assignors to The Dow..(;3h'emioal Com'- 'pany, Midland, Mich., a corporation of ela-;

No Drawing: Appi'ieetiehseptember 6.1949,- v Serial No. 1142290 4 Qlaiil'ls; (otffatz msj;

*Thinvention relates to a" method of reniov- Table ing calcium sulfate scale deposits from heat exp, p 4y change surfaces. 1 i

One of the most common kinds of scale which Scale sment' deposits upon heat exchange surfaces involved in 5 g3gfggfi heating and boiling many natural waters is cal- Watepsduble Chmmium ttfliffiigi g Solvent cium sulfate or anhydrate in various states of Cmpmmd Grams Solvent purity. Its presence is usually detrimental to the efficiency of the apparatus on which it deposits in Crow 0.010s

, (NH4)ZCI'O4..--. 0.049 0.00465 use and much time, labor, and inconvenience are K c m 0.05 000449 involved in its removal by conventional methods NEMCYOA 00 0.00403 KzCr2O1 0.025 0.00386 WhlCh are generally limited to mechanically dis- N M25 M0401 lodging the deposits. Insofar as we are aware, no (NHO2CHO1 .0 5 0. 00402 satisfactory methods are commercially available by which calcium sulfate scale can be dislodged Similar tests were made on an impu ywithout the expenditure of an inordinate amount drite scale containing 50 per cent by weight of cal-.

of ti a d 13,1 cium sulfate, 15 per cent of alpha quartz, the re- We have discovered that by subjecting the .mainder unidentified siliceous material, with the calcium sulfate scale to the dissolving action of a fo lowi g results tabulated in Table 11.

hot aqueous solution of certain water-soluble iO Table H chromium compounds, the scale is readily dissolved and dislodged so that a clean surface is G to so obtained with substantial savings in time and Aqueous Solution Used 1355 5 55180 psr 10$ labor. Grams Solvent The water-soluble chromium compounds whose aqueous solutions we have found dissolve calcium 5% 011mm. 1 -540 10% chromic mtrate 1 37 sulfate scales are the followmg: Chronnc acid 15% chromic bromide 2.104

(CrOa dissolved in water), the alkali metal and ammomum ehromatesend dlehromatesl Phmmlc In the tests of Table II, the chromium com-.- brOmlde, chromic chlorlde, chromlc mtmtepound was dissolved in water in the concentration These Water Soluble chromlum compounds are by weight indicated and the solution was maina s g z at m ie molar up tained at 210 F. in contact with the scale for 3 0 g in gg f n th g s zf fi grfifi 1 22 1 55: hours. After acting upon the scale, the spent 096 0 e e c c c as solvent was separated from the undissolved porg ggfig g f i ig i i g i tion of the scale and analyzed for its calcium sulfate content which is set forth in t la creased as the temperature of the solution is inf the tabla he St column creased. A generally suitable operating temperature is 150 F. although it is preferable to employ EXAMPLE 1 the solution at its m p e boiling P 40 -A boiler has a liquid capacity of 2000 gallons and The length o t h 50111131010 y be allowed has deposited on the steam generating surfaces 5: P e Scale dependstupon t ze t s s 250 pounds of anhydrite. 830 pounds of chromic o 6 Se 8 d Opera ing ampere l1 acid are dissolved in enough water to make 2000 $333121. tifi fiitivifii$3331.22??? 33 .3512 t ift t r; t I ion w 10 is in r0 uce in 0 he boiler and tam the solution in contact with the scale for at heated t i t boiling, h l t i k t lleaabstFl, to 6 hours at a temp of at least boiling for about6 hours then drained and flushed.

The scale is removed. The following tabulated examples are illustrative of the dissolving action of the solutions on EXAMPLE 2 calcium sulfate scale (anhydrite) the solution be- A heat exchanger has a liquid capacity of 246 ing held for 3 hours at the boiling temperature in gallons and the heat exchange surface has decontact with a large amount of anhydrite. The posited thereon 12 pounds of calcium sulfate scale. data show the weight of anhydrite dissolved by a As seen in Table I, 100 grams of a 0.025 molar solugiven weight of chromium compound under the t10n of sodium dichromate (NazCmO'z) dissolves e ified conditions 0.004 mole of calcium sulfate. Using this data,

it is calculated that 164 pounds of sodium dichromate dissolved in water sufilcient to fill the heat exchanger are used. The solution is heated to boiling and introduced into the heat exchanger and kept hot in the exchanger for 3 hours. The resulting spent'solution is drained out and the exchanger flushed with water. The exchanger thus freed of scale is ready for use.

We claim:

1. In a method of removing calcium sulfate scale deposited upon a heat exchangesurface, thesteps which consist in subjecting the deposit to the dissolving action of an aqueous solution of a water-soluble chromium compound selected from the group consisting of chromic acid, allsali metal and ammonium chromates and dichromates, chromic chloride, chromic bromide, and chromic nitrate, said compound being present in the solu tion in a concentration of at least 0.025 mole per 100 grams of the solution and the solution being heated to a temperature between about 150 F. and its atmosphere boiling. point.

REFERENCES CITED The following references are of record in the file of this patent:

FOREIGN PATENTS Number Country Date 331,725 Great Britain July 10, 1930 491,023

Great Britain Aug. 24, 1938 

1. IN A METHOD OF REMOVING CALCIUM SULFATE SCALE DEPOSITED UPON A HEAT EXCHANGE SURFACE, THE STEPS WHICH CONSIST IN SUBJECTING THE DEPOSIT TO THE DISSOLVING ACTION OF AN AQUEOUS SOLUTION OF A WATER-SOLUBLE CHROMIUM COMPOUND SELECTED FROM THE GROUP CONSISTING OF CHROMIC ACID, ALKALI METAL AND AMMONIUM CHROMATES AND DICHLORMATES, CHROMIC CHLORIDE, CHROMIC BROMIDE, AND CHROMIC NITRATE, SAID COMPOUND BEING PRESENT IN THE SOLUTION IN A CONCENTRATION OF AT LEAST 0.025 MOLE PER 100 GRAMS OF THE SOLUTION, AND THE SOLUTION BEING HEATED TO A TEMPERATURE BETWEEN ABOUT 150* F. AND ITS ATMOSPHERE BOILING POINT. 